This invention relates to bone penetrating pins and associated devices for applying and limiting torque during the installation thereof. Such pins are used in various medical procedures, including, but not limited to those relating to the cervical spine and skull. For instance, a patient's head must be held in a fixed, predetermined position while a fracture or dislocation of the cervical spine is healing, by utilizing a conventional halo traction device or, during neurosurgery, utilizing an operating table head holder. While the patient's head is being immobilized, it may also be desirable to subject the patient's skull to an imaging procedure, such as a CT scan, digital subtraction angiography, sonography, magnetic resonance imaging, computer enhanced flat film X-raying, and the like. Therefore it is highly desirable that the affixation apparatus be made of a material which does not artifact significantly, i.e., to any greater extent than bone matter.
It is also desirable during a number of other surgical procedures to quickly and accurately insert fastener pins in bone material.
It is also desirable, during such procedures, to be able to install such pins without utilization of complex, specialized, and consequently expensive tooling and the like.
In applicant's prior U.S. Pat. No. 4,612,930, skull pins, which are constructed in such a way that they do not artifact significantly, and related crown apparatus are disclosed which avoid any interference with imaging, but which nevertheless provide effective skull fixation. The subject matter of this earlier patent is incorporated herein by reference.
According to this invention, torque applying and torque limiting devices are disclosed which enable quick, precise and accurate installation of bone penetrating pins.
In one embodiment of the invention, the torque applying and limiting functions are embodied in a cap which may be detachably mounted with respect to the pin itself.
In this first embodiment, the pin includes a bone engaging portion which terminates in a pointed tip, and which is composed of a first material, such as ceramic or titanium. Most desirably the bone engaging portion is of a single crystal ceramic material, such as single crystal alumina ceramic, commercially sold under the trademark "BIOCERAM" of Kyocera International, Inc. of West Lost Angeles, Calif. This material does not artifact any more than bone matter; however, it has a brittleness which mitigates against formation of the entire pin of the material. Therefore, the bone penetrating pin according to the invention also comprises an exteriorly threaded, elongated cylinder or shank, of a second material that is radiolucent, such as plastic or aluminum. Preferably, the second material is a plastic. The bone engaging portion is fixed to the plastic or aluminum cylinder in any suitable manner. For example, the bone engaging portion may be formed with a rearwardly extending shaft received within an opening provided in an end face of the elongated cylinder. If the cylinder is plastic, it may be molded about the shaft extension of the bone penetrating portion. If the cylinder is metal, the shaft extension may be secured within the cylinder by epoxy adhesive or the like. The cylinder is provided at its opposite end with a blunt, driving end, and preferably having a screwdriver blade-receiving slot formed therein.
In accordance with this first embodiment of the invention, a removable plastic cap is utilized, which fits over the blunt driving end of the pin, and which is designed for performing the dual function of applying the torque necessary to install the pin, and of preventing an over-torqued situation by failing upon the application of torque greater than a predetermined maximum.
The removable cap is formed as a single element, comprised of upper and lower cylindrical portions separated by an intermediate, cylindrical portion of smaller diameter than the upper and lower portions. Torque applying means, in the form of outwardly extending "wings" or "arms" are integrally formed within the upper portions of the cap. The lower portion of the cap is provided in its lower end face with a bore having a peripheral shape complimentary to the shape of the blunt, driving end of the cylinder. In a preferred embodiment, the blunt driving end and lower cap bore have substantially square shapes, although other shapes, e.g., hexagonal, may also be employed.
It will be understood that torque applied at the upper portion of the cap is transmitted to the lower portion via the intermediate portion. This intermediate cylindrical portion of smaller diameter is designed to fail upon the application of torque exceeding a predetermined maximum level. When the maximum level is exceeded, the upper portion of the cap shears off at the intermediate portion, and is rendered inoperable for torque applying purposes. In such case, when it is desired to remove the pin, a screwdriver or similar tool may be utilized in conjunction with the slot in the upper end face of the cylindrical shank, which is exposed upon removal of the upper portion of the cap.
In a further aspect of the invention, the penetrating pins are utilized as skull pins in conjunction with a halo crown of the type described in my previously mentioned prior patent.
In another embodiment of the invention, the torque applying and limiting means are in the form of a tool assembly comprising a shank engageable with one end of a bone penetrating pin and a cap slidably mounted with respect to the shank, wherein a torque limiting insert is receivable between the cap and the shank to drivingly couple the two. More specifically, the tool of this second embodiment includes an elongated shank provided with a pin fastener engaging tip at one end and a first socket formed at the other end. A torque applying device is slidably mounted with respect to the shank, and includes a second socket substantially aligned with the first socket. A torque limiting insert is receivable within the first and second sockets to couple the torque applying device and the pin engaging shank.
The torque limiting insert is provided with an upper portion having a cross-sectional shape complimentary to that of the second socket; and a lower portion having a cross-sectional shape complimentary to the first socket. The upper and lower portions of the insert are interconnected by an intermediate, grooved area which is designed to fail in shear upon the application of torque beyond a predetermined maximum.
Other objects and advantages of the invention will become apparent from the detailed description of the invention which follows hereinbelow.